1. Field of the Invention
The present invention relates to an output circuit employing feedback control, a liquid crystal driving circuit that uses the output circuit to drive a liquid crystal panel, and a liquid crystal driving method that uses the output method of the output circuit to drive a liquid crystal panel.
2. Description of the Related Art
As disclosed in Japanese Unexamined Patent Application Publication No. 11-30975, the driving speed of a liquid crystal display panel having source lines driven by operational amplifiers can be increased by precharging the source lines. The source lines are precharged by disconnecting them from their drivers (the operational amplifiers) and either interconnecting the source signal lines, or connecting them to a fixed potential such as the common-voltage potential of the liquid crystal display panel.
FIG. 8 illustrates the former precharging scheme in a conventional liquid crystal display including a liquid crystal panel 1, a gate driving circuit 2, a source driving circuit 3, a group of m source lines S1, S2, . . . , Sm, and a group of n gate lines G1, G2, . . . , Gm, where m and n are positive integers, m being equal to or greater than two. The liquid crystal panel 1 includes cell transistors TRij and capacitors CXij (1≦i≦m, 1≦j≦n). The gate driving circuit 2 includes gate drivers GDj (1≦j≦n).
Referring to FIG. 9, the source driving circuit 3 comprises m source drivers SD1, SD2, . . . , SDm, connected through respective analog switches A1, A2, . . . , Am to respective output terminals OUT1, OUT2, . . . , OUTm, a group of m−1 analog switches D1, D2, . . . , Dm−1 by which mutually adjacent source lines are switchably interconnected, and an inverter I. A single output circuit comprises a source driver SDi, the corresponding analog switches Ai, Di, and output terminal OUTi (where i is an arbitrary integer from 1 to m). The source driver SDi is an operational amplifier receiving a source driving signal SSi as its non-inverting input, generating a corresponding output signal for driving source line Si, and feeding the output signal back as its inverting input. Feedback ensures that the output signal has the same potential as the source driving signal SSi. Various other impedance conversion means controlled by feedback can also be used as the source driver SDi.
Analog switches A1 to Am and D1 to Dm−1 are controlled by a switch control signal PC input to inverter I and a complementary switch control signal PCB output from inverter I. When switch control signal PC is ‘0’ and PCB is ‘1’, analog switches A1 to Am all turn on and analog switches D1 to Dm−1 all turn off, so that output terminals OUT1 to OUTm (and source lines S1 to Sm) are connected to the output terminals of respective source drivers SD1 to SDm and the output signals from the source drivers SD1 to SDm are output on source lines S1 to Sm. When switch control signal PC goes to ‘1’ and switch control signal PCB goes to ‘0’, analog switches A1 to Am all turn off and analog switches D1 to Dm−1 all turn on, disconnecting output terminals OUT1 to OUTm (and source lines S1 to Sm) from the source drivers SD1 to SDm and interconnecting all of the output terminals and source lines; the output terminals and source lines are thereby precharged. When switch control signal PC returns to ‘0’ and switch control signal PCB returns to ‘1’, analog switches A1 to Am all turn on and analog switches D1 to Dm−1 all turn off, disconnecting output terminals OUT1 to OUTm (and source lines S1 to Sm) from each other and connecting them to the source drivers SD1 to SDm.
Although the purpose of this precharging scheme is faster driving, to enable the source drivers to receive feedback during the precharging period, the feedback signals must be taken from points between the source drivers and the analog switches A1 to Am. Consequently, during driving periods, the source drivers must drive the on-resistance of these analog switches as well as the capacitance of the capacitors in the liquid crystal panel. Because of the voltage drop due to the on-resistance of the analog switches, the potentials of the output terminals of the source driving circuit 3 differ from the potentials of the signals output by the source drivers. Although the potential difference diminishes and eventually disappears as the capacitors approach and eventually reach the intended charge level, the potential difference slows the approach, thereby limiting the speed with which the liquid crystal panel can be driven. A further problem is that variations in wiring resistance due to variations in the on-resistance of the analog switches and the wiring length of the output paths create unwanted variations in driving potential among the output terminals (and source lines), impairing the accuracy with which the liquid crystal panel 1 is driven, leading to lowered image quality. As the number of pixels increases and the driving frequency increases, driving the liquid crystal panel accurately at the necessary speed becomes a significant challenge.